1. Field of the Invention
The present invention relates to the operation of a dry-type electrical transformer, and more particularly relates to the use of air to cool a dry-type electrical transformer.
2. Background of the Related Art
A transformer is a device that transfers electrical energy from one circuit or winding to another through inductively coupled conductors. Varying current in a primary winding creates a varying magnetic flux in a ferromagnetic core of the transformer, which results in a varying magnetic field through a secondary winding. This varying magnetic field induces a varying voltage in the secondary winding. When a load is connected to the secondary winding, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the relationship between the induced voltage in the secondary winding (VS) and the voltage in the primary winding (VP) and is given by the ratio of the number of turns in the secondary winding (NS) to the number of turns in the primary winding (NP), according to the follow equation:
            V      S              V      P        =            N      S              N      P      Accordingly, a transformer enables an alternating current (AC) voltage to be increased by making NS greater than NP, or decreased by making NS less than NP.
However, the conductive metal wire used in the windings of the transformer imparts resistance to the electric current. A byproduct of this resistance is the production of heat within the metal wire. Unfortunately, the electrical resistance of the metal increases with rising temperature. Because the higher resistance will cause the production of even more heat and further reduce the efficiency of the transformer, it is important to remove heat from the transformer windings.
Some transformers are cooled by disposing them in a tank filled with oil that circulates through a radiator by natural convection or forced circulation. Other “dry type” transformers contain no liquid and are cooled by one or more gases. For example, in a ventilated dry type transformer, the transformer assembly is cooled by natural convection air currents or by forced air cooling.